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The massive, star-studded cast also talked to THR about how director Bryan Singer's familiarity with the X-Men universe helped with this movie.

As the cast of X-Men: Days of Future Past tries to change the past to save the future, the fates of the younger versions of Magneto, Professor Xavier and Mystique, introduced in X-Men: First Class, are somewhat up in the air at the beginning of the movie.

Jennifer Lawrence laid out the central question for her character when she spoke with The Hollywood Reporter on the black carpet ahead of Days of Future Past's global premiere in New York Saturday night.

"This time because the future is undetermined, we have freedom to choose where we want to go with her," the actress explained. "Is she going to become the assassin that we know her as now, like Rebecca Romijn's character? Or is she going to do what Charles [Xavier] wants and go down the path of no killing?"

Of the younger X-Men characters, James McAvoy's Xavier is the only one who gets to meet his older counterpart (played by Patrick Stewart) in the film.

As he was able to watch Stewart's performance up close, McAvoy said there was one element of the veteran actor's portrayal of Xavier he wanted to replicate.

"The biggest thing I took away from his portrayal of Charles was his empathy. It poured out of him," McAvoy told THR. "It needs to pour out of the character, but it also pours out of him as an actor and a performer and that was something I wanted to take into my performance as well."

But McAvoy explained that it was important that his version of Xavier be different from Stewart's, which led him to try to ignore the older character.

"I think the job of doing First Class and even doing this one was not redefining him for the sake of it and not just because it's a reboot but because you're showing him at a different time of his life," McAvoy explained. "If he was just the same it would be deeply uninteresting. So it was important to me to find a reasonable and believable different take on him, and ultimately I'll be much more like him and moving towards his performance."

As for Michael Fassbender, he found his own way to connect with Ian McKellen's Magneto even though the two actors weren't able to meet until Comic-Con.

"Through YouTube, I found a clip of him where he's doing an RSC workshop talking about how to approach Shakespeare, so I watched that. It was about 10 minutes long, and it was filmed in the '70s, so it was a younger McKellen," the actor said. "So I just spent a lot of time with that, so I felt like I was with him, as it were."

The massive cast of Days of Future Past includes the younger X-Men from First Class and many of the actors from the original wave of X-Men films, including Hugh Jackman, Halle Berry and Anna Paquin. But two familiar actors from the first wave of X-Men films appeared on the Days of Future Past carpet even though it was unclear whether they would appear in the current movie: Famke Janssen (Jean Grey) and James Marsden (Cyclops).

Both stars were coy on the carpet, with Marsden simply saying he was there to support the franchise, but they were brought on stage as part of the film's large cast when 20th Century Fox Film head Jim Gianopulos introduced the movie's stars ahead of the screening.

During his remarks, Gianopulos thanked director Bryan Singer, who wasn't on hand after announcing he was pulling out of all Days of Future Past publicity due to the sex abuse allegations he's facing.

And while Singer's current scandal wasn't mentioned at the premiere, castmembers talked to THR about how his familiarity with the X-Men universe, having directed the first X-Men film in 2000 and X2, helped with this movie.

Stewart said Singer was responsible for so much with the X-Men films, including "the tone, the mood, the style, the quality of it."

"And to have Bryan coming back for this incarnation meant everything because he has always had the content of the movie at heart as well as the spectacle," Stewart added.

THR cover star Ellen Page indicated that Singer's familiarity with the characters was reassuring heading into an ambitious story.

"I think reading the script we were just like, 'Wow this is so ambitious,' but you just knew he was going to pull it off because he has such a vision and is so meticulous and so assured," she said. "You watch him work, and it's a little bit mind-boggling how he manages to handle it all."

The ambitiousness of the script partly included continuing the storylines from First Class and reintroducing the stars from the trio of X-Men movies that ended with 2006's The Last Stand.

Producer-writer Simon Kinberg told THR that it was so challenging to keep everyone's storyline straight that he actually made up color-coded notecards for each character.

"Having you know eight to 10 main characters that you have to take care of, any two or three of which have old and young versions of the character, the way I kept it straight was I actually created colored notecards," he said. "I had a different color I assigned to each character, and I kept the timelines for each character up on a bulletin board wherever I was, whether I was at the production office in Montreal or at home in L.A., and I would just reference that and make sure that I was always taking care of each storyline."

Kinberg explained that as both a writer and producer, the latter makes him "slightly more responsible as a writer" in that he's aware of the technical and logistical realities of what he writes.

"I'm still kind of irresponsible as a writer and ultimately do things that like the producer side of me is like, 'We can't afford' or 'We can't figure out,' " he said.

Longtime X-Men producer Lauren Shuler Donner has been talking about making Days of Future Past for years, but she explained that this was the right time to do it.

"It's the right time because we didn't want to just do a sequel to First Class. It didn't feel like we could be original enough," she said. "And we all miss … Patrick and Ian and Halle, everybody from the older cast, so we just felt like let's do this and that way it will bring everybody together."

When asked if there were other X-Men stories she's itching to tell, she simply said "yeah," but couldn't reveal further details except to hint that one secret film was in the works.

"There's one I'm really itching to tell you about but I can't," she told THR. "You're gonna like it though. I guarantee you. It's really different, but you're going to like it."